1. Field of the Invention
The invention generally relates to surface acoustic wave devices and methods of fabricating the same, and more particularly, to a surface acoustic wave device having a SAW chip that is attached to a first substrate and is sealed with a second substrate and a method of fabricating the same.
2. Description of the Related Art
Recently, there has been a demand to downsize electronic elements mounted to electronic devices and improve the performance thereof with downsizing and high performance of the electronic devices. For instance, there have been similar demands on surface acoustic wave (SAW) devices that are electronic parts used as filters, delay lines, oscillators in electronic devices capable of transmitting and receiving radio waves.
A description will now be given of a filter device equipped with a conventional SAW device. FIG. 1A is a perspective view of a SAW filter 100, and FIG. 1B is a cross-sectional view taken along a line D-D shown in FIG. 1A. This type of SAW device is disclosed, for example, Japanese Patent Application Publication No. 8-18390 (see FIG. 4, particularly).
Referring to FIG. 1A, the SAW filter 100 includes a ceramic package 101 having a cavity 102, a metal cap 103 and a SAW chip 111. The SAW chip 111 is placed in the cavity 102, which is sealed with the metal cap 103. As shown in FIG. 1B, the package 101 has a three-layer structure composed of three joined substrates 101a, 101b and 101c. Electrode pads are provided on the top of the substrate 101b, and foot patterns 104 are provided on the bottom of the substrate 10c. Wiring patterns are provided on sides of the package 101, and connect the electrode pads and the foot patterns 104. The SAW chip 111 is fixed to the bottom of the cavity 102 so that comb-like electrodes (an interdigital transducer: IDT) on the SAW chip 111 face up. Electrode pads on the SAW chip 111 are connected to the pads via metal wires 112. The metal cap 103 is bonded to the top surface of the package 101 by a bonding material 105 made of solder or resin.
There is another proposal to mount the SAW chip in flip-chip fashion (see, for example, Japanese Patent Application Publication No. 2001-110946). FIGS. 2A and 2B show this type of SAW device. More particularly, FIG. 2A is a perspective view of a SAW chip 211 of a SAW filter 200, and FIG. 2B is a cross-sectional view of the SAW filter 200, which view corresponds to a cross section taken along the line D-D shown in FIG. 1A.
As shown in FIG. 2A, the SAW chip 211 has a piezoelectric material substrate (hereinafter referred to as piezoelectric substrate) 212. Comb-like electrodes 213 that form an IDT are formed on a main surface (upper surface) of the piezoelectric substrate 212. Electrode pads 214 are provided on the main surface and are electrically connected to the IDT 213 via a wiring pattern. As shown in FIG. 2B, a package 201 has a cavity 202. Electrode pads 205 are provided on the bottom of the cavity 202, which is also referred to as die-attached surface. The pads 205 are positioned so as to correspond to the pads 214 of the SAW chip 211. The SAW chip 211 is flip-chip mounted in the cavity 202 so that the IDT 213 and the electrode patterns 214 face the die-attached surface. The pads 214 and 205 are bonded via metal bumps 215 so that these pads are electrically and mechanically fixed together. The pads 205 are electrically connected to foot patterns 207 on the backside of the package 201 by means of via-wiring lines 206, which penetrate the bottom portion of the package 201. A metal cap 203 closes an opening of the cavity 202 and is bonded to the package 201 by a bonding material 204.
A duplexer equipped with a transmit filter and a receive filter may be formed by using SAW filters as mentioned above. Such a duplexer will now be described with reference to FIGS. 3A and 3B. A duplexer 300 shown in these figures has a transmit filter 311a and a receive filter 311b, each of which filters is like the SAW filter 100. FIG. 3A shows a cross section view of the duplexer 300, which corresponds to that taken along the line D-D shown in FIG. 1A. FIG. 3B is a plan view of a SAW chip 311.
Referring to FIG. 3A, the duplexer 300 has a package 301 in which the SAW chip 311 is mounted. A matching-circuit board 321 and a main board 322 are provided on the bottom side of the package 301. The matching-circuit board 321 is provided in such a way as to be sandwiched by the main board 322. As shown in FIG. 3B, the SAW chip 311 is equipped with the transmit filter 311a and the receive filter 311b. Each of the filters 311a and 311b has respective IDTs 313 arranged in ladder fashion. The IDTs 313 are connected to electrode pads 314 via wiring patterns 315.
The SAW filter or duplexer as mentioned above is required to have the SAW chip hermetically sealed. The metal cap is used, along with bonding material or resin, to accomplish hermetical sealing.
However, there are drawbacks to be solved. A large joining area (seal width) at the interface between the package and the cap is needed to hermetically seal the cavity with high reliability. However, this prevents downsizing of the package. Downsizing of package is also restricted due to the use of wires because the wires need a relatively wide pattern for bonding. The package is the multilayer substrate made of ceramics, which is comparatively expensive. The device needs the process of assembling the cap, chip and package device, and is therefore costly.